(1) Field of the Invention
This invention relates to a process for the denitration of exhaust gases, and more particularly to a denitration process which includes light irradiation to convert nitrogen dioxide into nitric acid.
More specifically, the present invention relates to a denitration process for exhaust gases in which the nitrogen oxide contained in the exhaust gas is oxidized with an oxidizing gas such as a chlorine dioxide gas (ClO.sub.2), ozone gas (O.sub.3) or the like to produce nitrogen dioxide (NO.sub.2), and the exhaust gas is then subjected to light irradiation in the presence of chlorine dioxide or ozone gas and nitrogen dioxide so as to convert the latter into nitric acid which may be recovered.
(2) Description of the Prior Art
Many processes have been proposed for removing nitrogen oxides (NO.sub.x) contained in various exhaust gases. These processes include: (A) washing the nitrogen oxide with an oxidizing solution such as a bleaching powder solution, a chlorous acid solution, or a hydrogen peroxide solution, (B) reacting the nitrogen oxide with an oxidizing gas such as chlorine dioxide, chlorine or ozone gas, and (C) treating the nitrogen oxide with a reducing solution such as sodium sulfite solution or a mixture of an organic chelate compound solution and a sodium sulfite solution.
These processes possess a variety of advantages and disadvantages and difficulty is encountered in determining their superiority from an economic viewpoint. For instance, process (C) using a reducing solution is not recommended for exhaust gases from a sintering furnace or coal boiler, because a large amount of oxygen is contained in the exhaust gas. In addition, process (A) including oxidizing and absorbing steps by using liquid oxidizer suffers from its lowered efficiency, when applied to an atmosphere which does not produce an oxidizing gas, thus resulting in a NO.sub.x -removal rate in the order of at most 40 to 70%. In contrast thereto, process (B) using an oxidizing gas, particularly, chlorine dioxide gas or ozone gas provides a strong oxidixing ability and hence excellent efficiency in the removal of nitrogen oxide, although this process has some drawbacks.
For instance, in the case of oxidizing nitrogen monoxide (NO) with chlorine dioxide gas (ClO.sub.2), the following reaction occurs: EQU 2NO + ClO.sub.2 .sup.(H.sbsp.2.sup.O) NO.sub.2 (gas) + HNO.sub.3 + HCl
However, the chlorine dioxide gas (ClO.sub.2) does not have the capability to oxidize nitrogen dioxide gas (NO.sub.2), and thus only 50% of the nitrogen monoxide (NO) is oxidized. For this reason, it has been proposed to oxidize nitrogen dioxide (NO.sub.2) with a sodium sulfite solution (Na.sub.2 SO.sub.3). However, sodium sulfite (Na.sub.2 SO.sub.3) is consumed in considerable quantities due to the oxygen (O.sub.2) present in the gas, and in addition, the reaction mechanism nitrogen dioxide (NO.sub.2) with sodium sulfite (Na.sub.2 SO.sub.3) is complicated.
On the other hand, when oxidizing with ozone (O.sub.3) gas, the following reactions take place:
No + o.sub.3 .fwdarw. no.sub.2 + o.sub.2 (first stage oxidation) PA0 2NO.sub.2 + O.sub.3 .fwdarw. NO.sub.3 .multidot.NO.sub.2 + O.sub.2 (second stage oxidation)
The second stage oxidizing reaction has a low reacting rate, so that an excessive amount of ozone (O.sub.3) is required. In addition, a sodium sulfite solution (Na.sub.2 SO.sub.3) is required to treat the excess ozone (O.sub.3), which increases the ozone cost. For this reason, the above process is economically not desireable.